cmd: nand/sf: isolate legacy code

The 'sf' command is not supposed to rely on the MTD stack, but both
'sf' and 'nand' commands use helpers located in mtd_uboot.c. Despite
their location, these functions do not depend at all on the MTD
stack.

This file (drivers/mtd/mtd_uboot.c) is only compiled if CONFIG_MTD is
selected, which is inconsistent with the current situation. Solve this
by moving these three functions (which are only used by the above two
commands) out of mtd_uboot.c and put them in a C file only compiled
with cmd/sf.c and cmd/nand.c.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
[trini: Don't export get_part function now]
Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/cmd/legacy-mtd-utils.h b/cmd/legacy-mtd-utils.h
new file mode 100644
index 0000000..ac441d5
--- /dev/null
+++ b/cmd/legacy-mtd-utils.h
@@ -0,0 +1,12 @@
+/* SPDX-License-Identifier: GPL-2.0+ */
+
+#ifndef __LEGACY_MTD_UTILS_H
+#define __LEGACY_MTD_UTILS_H
+
+int mtd_arg_off(const char *arg, int *idx, loff_t *off, loff_t *size,
+		loff_t *maxsize, int devtype, uint64_t chipsize);
+int mtd_arg_off_size(int argc, char *const argv[], int *idx, loff_t *off,
+		     loff_t *size, loff_t *maxsize, int devtype,
+		     uint64_t chipsize);
+
+#endif /* LEGACY_MTD_UTILS_H */